1. Field of the Invention
The present invention relates to an image processing apparatus that can generate chart data to be used to correct colors of an output product printed by a printer. Further, the present invention relates to an image processing method, and a program capable of generating image processing parameters.
2. Description of the Related Art
Many electrophotographic apparatuses having been recently introduced have improved performances. For example, there is a conventional electrophotographic machine comparable to a printing machine in the capability of realizing higher image quality. However, the electrophotographic apparatuses are generally inferior to the printing machine in color variation amount that is caused due to unstableness peculiar to electrophotographic apparatuses.
Hence, the technique employed for a conventional electrophotographic apparatus to solve the above-described problem is the calibration using a one-dimensional gradation correction look-up table (LUT) that is dedicated to each of cyan, magenta, yellow, and black (key tone) (hereinafter, referred to as C, M, Y, and K) toners.
The LUT is a table that includes a predetermined number of output data corresponding to input data that are spaced discretely at specific intervals. For example, the LUT is usable to express non-linear characteristics that cannot be expressed using arithmetic expressions. The one-dimensional gradation correction LUT indicates output signal values of a printer that can express input signal values of C, M, Y, and K colors. The printer forms an image on paper using toners corresponding to the output signal values.
First, a chart including a plurality of pieces of patch data that are different in gradation is prepared beforehand for each of C, M, Y, and K toners and output by a printer. The value of each patch printed on the chart is read by a scanner or a colorimeter. Then, the read value (i.e., measurement data) is compared with target data stored beforehand. Thus, the one-dimensional gradation correction LUT can be generated independently for each of the C, M, Y, and K colors.
However, the one-dimensional gradation correction LUT applicable to the above-described gradation characteristics of a single color cannot assure the color reproducibility for a “mixed (or compound) color”, such as red, green, blue, and CMY-based gray, which can be formed by mixing a plurality of types of toners, because of non-linear differences that may be generated depending on individual printers.
Hence, as discussed in Japanese Patent Application Laid-Open No. 2006-165864, it is conventionally proposed to prepare a chart using mixed (or compound) colors in a region that can be reproduced by a printer, measure the chart data with a scanner or a colorimeter, compare the measured values with target values (target data), and generate correction values.
The International Color Consortium (ICC) provides ICC profiles as data usable in color conversion. It is conventionally known that color differences of mixed (or compound) colors can be corrected by amending a destination profile included in the ICC profiles.
Such a conventional method includes preparing a chart of mixed (or compound) colors generated by a printer, and measuring the chart data with a scanner or a colorimeter. The conventional method further includes generating difference data with reference to color measurement results and target values, updating, based on the generated difference data, a three-dimensional LUT (destination profile), which is used to convert a device-independent color space (L*a*b*) included in the ICC profiles into a device-dependent color space (CMYK), and correcting the mixed (or compound) colors based on the updated three-dimensional LUT. The color space L*a*b* is one of the device-independent color spaces. The value L* represents “luminance”, the value a* represents “hue”, and the value b* represents “saturation.”
Further, it is conventionally known to provide a built-in sensor on a sheet conveyance path extending from a sheet fixing position to a sheet discharge position in the printer, instead of using a scanner or an externally connected colorimeter. A system equipped with the built-in sensor can read a gradation correction chart while the chart is conveyed along the sheet conveyance path after passing through a fixing device and before exiting from the printer.
After a calibration CMYK chart is printed and fixed, the chart data is read by the sensor. The read values are compared with target data prepared beforehand, and a one-dimensional correction LUT can be generated. According to the system equipped with a color-value measuring sensor, the sensor is fixed at a predetermined position relative to a sheet (i.e., a target to be read) being conveyed along the sheet conveyance path. Therefore, the number of patches (i.e., chart data) on the chart can be increased only in the sub scanning direction (i.e., sheet conveyance direction).
To ensure the sensor is able to read patch data on a chart, each patch to be printed on the chart is required to have a certain level of size. Therefore, the total number of pieces of chart data that can be disposed on a sheet is limited. Therefore, compared to the method using a scanner, the method employing a built-in sensor requires generating a number of charts to enable the sensor to read a sufficient number of patches. The amount of paper consumption thus increases greatly. Therefore, a conventional system is configured to increase the number of sensors mounted on a single machine to increase the number of pieces of chart data that can be read on a sheet of paper.
However, when a plurality of sensors is used for a single calibration, the accuracy of reading values decreases significantly because of individual differences of respective sensors. Therefore, the final calibration accuracy decreases correspondingly. According to the correction using the one-dimensional LUT, an independent sensor is allocated to each of C, M, Y, and K toners. When the same toner patch is read by the same sensor, influences of the differences caused by individual sensors can be suppressed adequately. However, the “mixed (or compound) color patch” that can be formed by mixing a plurality of types of toners tends to be influenced greatly by the differences of individual sensors, because matching the toner with the sensor is not easy. As described above, in the calibration of “mixed (or compound) colors” having various hue/lightness/saturation characteristics, adverse influences may arise because of the differences of individual sensors.